1. Field of the Invention
The present invention relates to a stator core, a manufacturing method therefor, and a rotary electric machine that uses that stator core, and particularly relates to a stator core that is formed by integrating laminated electromagnetic steel sheets by welding.
2. Description of the Related Art
Conventional stator cores that are used in automotive alternators are formed into an annular shape by laminating a predetermined number of magnetic segments that are punched into predetermined shapes from a magnetic steel sheet by pressing, forming a rectangular parallelepiped laminated core by laser welding outer circumferential portions of the laminated body of magnetic segments, bending the rectangular parallelepiped laminated core annularly, abutting two end surfaces of the bent laminated core, and laser welding the abutted portions thereof (see Patent Literature 1, for example).
Now, organic insulating coatings are coated onto surfaces of the magnetic steel sheet in order to ensure punchability, electrical insulation, and corrosion prevention among the magnetic segments. Thus, organic substances in the insulating coating decompose thermally when the outer circumferential portions of the laminated body of magnetic segments are laser-welded, giving rise to volatile gases. If these volatile gases penetrate into the weld bead, pits and blowholes occur in the weld portion. These pits and blowholes that occur in the weld portion give rise to reductions in weld strength.
Magnetic steel sheets include silicon steel sheets, electromagnetic steel sheets, etc., but in recent years, electromagnetic steel sheets are being used from the viewpoint of reducing core loss. In addition, from the viewpoint of environmental protection, materials that do not contain substances of environmental concern, such as chromium, are used in materials for the electromagnetic steel sheets, and materials that contain organosilicon compounds (silicones) are used in the insulating coatings. Since combustion temperatures of silicones in particular are high, the weld portions harden before the gases formed by combustion finish passing through the weld pool, and welding faults such as pits or blowholes occur easily.
In consideration of such conditions, various countermeasures have been proposed that prevent entry into a weld bead of the volatile gases that are generated by thermal decomposition of the organic substances in the insulating coatings. In the technique according to Patent Literature 2, an electromagnetic steel sheet is used that is formed so as to have a first surface that is a rough surface and a second surface that is a smooth surface, and penetration of gases into the weld bead is stopped by forming air gaps between the magnetic segments by stacking the punched magnetic segments such that the rough surfaces and the smooth surfaces face each other such that the gases that originate from the insulating coating are dispersed through those air gaps during welding. In the technique according to Patent Literature 3, gas drainage channels are formed on the weld portions by pressing, and penetration of gases into the weld bead is stopped by dispersing the gases that originate from the insulating coating through those gas drainage channels during welding.
Patent Literature 1: Japanese Patent Laid-Open No. 2002-159151 (Gazette)
Patent Literature 2: Japanese Patent Laid-Open No. HEI 1-232705 (Gazette)
Patent Literature 3: Japanese Patent Laid-Open No. 2011-87386 (Gazette)
However, in the technique according to Patent Literature 2, because an electromagnetic steel sheet is used in which a first surface is formed into a rough surface, one disadvantage has been that magnetic properties and space factor decrease. Because hardness increases if the silicon content of the electromagnetic steel sheet is increased, making it hard to form the rough surfaces, another problem has been that grade constraints are placed on the electromagnetic steel sheet.
In the technique according to Patent Literature 3, because the gas drainage channels are formed on the weld portions, machining strain is generated around the gas drainage channels, and one disadvantage has been that magnetic properties decrease.